1. Field of the Invention
The present invention relates to an automatic focusing device used in a photographing device to automatically focus a photographed image.
2. Description of the Related Art
In general, a trigonometrical survey method, an infrared ray system, etc. were used in an automatic focusing device used in a photographing device such as a video camera. However, recently, many automatic focusing devices have directly used a signal transmitted from an image pickup device for converting a photographed image to an electric signal in a focusing operation. Such a system is generally called a mountaineering system. This system uses a nature in which the amplitude of a high frequency component of a video signal transmitted from the image pickup device is highest when the photographed image is focused most suitably. A band-pass filter (which is called BPF) is used to obtain this high frequency component and the high frequency component is passed by the band-pass filter from the above video signal.
However, there is a case in which the high frequency component is temporarily highest at a certain timing although no photographed image is focused. Such a phenomenon is called a false peak. There are various kinds of methods for avoiding the false peak. For example, there is a method in which amplitudes of some kinds of high frequency components are simultaneously measured and a true focusing point is judged when all peaks of the high frequency components are in conformity with each other. In this method, it is necessary to use several kinds of band-pass filters so that the number of parts is increased. Further, there is dispersion with respect to heights, widths, etc. of the above peaks in accordance with the photographed image. Therefore, there is a problem about a measuring range of the high frequency components. It is ideal to measure many frequency components and select a most suitable range of the frequency components in accordance with the photographed image. However, in such a method, the number of parts is increased as mentioned above and manufacturing cost is increased.